Support Python 3.13, add Django 5.1 trove classifier, and fix Python 3.7 CI

[johnthagen]

Description

• Add support for Python 3.13
• Add Django 5.1 trove classifier to signal support, following
  • fixes for Django 5.1, add support for new STORAGES setting #524
• Pin to an older Ubuntu version in CI to allow testing under Python 3.7, which is required until @jezdez transfers this project out of jazzband so that we can admin the required CI checks and drop 3.7.
  • Transfer Out: django-dbbackup help#336

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.44%. Comparing base (8f1307e) to head (7225b69).
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #543      +/-   ##
==========================================
+ Coverage   91.40%   91.44%   +0.03%     
==========================================
  Files          19       19              
  Lines         873      912      +39     
  Branches      157      154       -3     
==========================================
+ Hits          798      834      +36     
- Misses         40       43       +3     
  Partials       35       35              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[johnthagen]

@Archmonger Please squash this when you merge.